ibook G4 wont boot up

I have a G4 Ibook, been having a problem with it about a week where it wont boot up, it switches on, makes the apple noise and even gets to the screen where the apple logo is showing with the timer at the botto,. problem is it stays there!
Before i would turn off and on and try a few times and i would eventually get lucky, but obviously something is not right.
What can i do? and if i can get it on what programme shall i run to check it?

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